The invention relates to a two-dimensional (2D) touch panel, more especially a touch panel that operates with capacitive touch sensing.
2D touch panels based on capacitive sensing are in widespread use in a variety of applications. Typically, the 2D touch panels are actuated by a finger, either in direct contact, or through proximity (i.e. without contact). For example, they are used as touchpads of laptop computers, in control panels of domestic appliances such as microwave ovens and cooktops, and as overlays to displays on hand held devices, such as mobile telephones. Many other applications are known in the art.
Most conventional designs of 2D touch panel are designed in a way which means that the touch panel can only detect a single touch at any one time. This is adequate for a large range of applications. However, for some applications, it is desirable for a 2D touch panel to be able to sense two or more touches simultaneously.
For example, as is well known, computers are conventionally controlled by a mouse, which combines two or three sensors, namely the tracking ball for the cursor motion and two buttons for selection of icons at the cursor position. A mouse thus combines cursor motion through movement of the mouse device, and two finger actions for actuating the left and right mouse buttons. In a laptop, the mouse functions are provided by a touchpad with adjacent buttons. A user moves the cursor through sliding one finger over the touchpad area, and selects icons and so forth by actuation the two “mouse” buttons with his or her thumb, or one or two other fingers.
Another example of a device that requires multiple simultaneous finger inputs is a hand-held games console, where typically the left and right thumbs are used to control different functions of the device, or jointly to control the same function. Controllers for in-flight entertainment systems often have a similar mode of operation.
A 2D capacitive touch panel capable of sensing multiple simultaneous touches is known in the prior art, and is now described.
FIG. 1 schematically summarises the prior art touch panel of U.S. Pat. No. 5,825,352 [reference 1] which is capable of detecting multiple touches simultaneously. An array sensor is formed by a touch pad matrix 101 with a plurality of wires extending in each of the x and y directions to form sensor line rows and columns respectively. The array sensor is scanned using a multiplexer 102 that is connected to the array sensor 101, which is controlled by a microcontroller 105. The capacitance of each of the sampled x and y wires is then measured using capacitance measuring circuitry 103. For calibration purposes, the sensor scans itself during a period when no finger actuations are present to determine a background signal level. The background capacitance level is measured and stored, and then subtracted from each scan of the sensor array, to determine the finger-induced capacitance. The scanned outputs from each sensor row and column in the sensor array are converted into a digital representation using an analogue-to-digital converter (ADC) 104 which supplies the digitised signals to the microcontroller 105. The scanned data are analysed in the x-direction and the y-direction of the array, either sequentially or concurrently. In the figure, the x-profile 107 and the y-profile 106 are illustrated schematically for a simultaneous touch of two fingers as illustrated on the array sensor 101. Referring to the x-profile 107, two maxima 108 and 110 are evident, one for each finger touch. The two maxima are separated by a minimum 109. Referring to the y-profile 106, only one maximum is evident, which is a result of the two finger touches being close together in the y-direction.
In this prior art device, the signals from the x-lines are analysed separately from the signals from the y-lines. In each of x and y, maxima and minima are identified, wherein maxima are designated as finger touches. A second maximum requires that a minimum is identified following the first maximum. Maxima are identified as the largest local variation in a signal. Minima are identified as a local minimum adjacent to a peak. After the maxima have been identified in each of the x and y directions, the value of each peak is compared to a threshold. If the value of the peak is less than the threshold it is no longer considered to be a peak. A similar function can be applied to the minima, whereby the value must be less than a given threshold value. The peak and valley data in the x and y directions are then interpolated to identify the location of one or more fingers on the sensor array. This technique is used for detecting and processing multiple simultaneous touches on a sensor array so that a first finger can be used to control a cursor (similar to a conventional touch pad on a laptop computer) and a second finger to provide actuations (similar to a conventional mechanical button provided adjacent to a touch pad).
While this prior art design provides a technically well-engineered solution for simultaneous multi-touch processing, it requires a large number of sensor lines in both x and y in order to provide sufficient spatial resolution. A minimum of approximately 10×10 lines would be required to resolve two simultaneous touches. More typically, perhaps at least 20×20 lines would be needed for sufficient reliability and accuracy, and to cope with more than two simultaneous touches. However, for mass-production devices, cost scales with the number of signal processing lines required, i.e. the number of lines in the capacitance measuring circuitry, ADC, and digital signal processor.
It would therefore be desirable to provide a multi-touch array sensor that could be constructed with a reduced number of sensor lines.